The present invention relates to an essentially closed press section of a paper machine for removing water from a paper web, the press section comprising a compact press roll combination in such a manner that its different rolls form several press nips, between which the web is guided to run principally supported by a roll surface without substantially long free draws.
Such press sections are generally used in paper machines in which there are several press nips through which the web and the water-removing fabric, such as a single or twin press felt, run. Reference is made to Valmet Oy Finnish Patent Applications Nos. 781,426; 821,995, to Valmet Oy U.S. Pat. No. 4,209,361, to Beloit Canadian Pat. No. 1,068,525, and to U.S. Pat. No. 4,075,056, as examples of the state of the art.
The Valmet Oy "Sym-Press II" (trademark) press section is known from U.S. Pat. No. 4,209,361. A press of this type has virtually become a standard fixture among various paper machine manufacturers. With this well-known press, several practially important advantages are attained. One of the most important advantages is that the paper web can be passed through the entire press section supported on a solid surface and as a closed draw, first between the first nip and the second nip on a pick-up felt over the sector of the press suction roll, and then through the third nip supported on the smooth surface of the center roll of the press which is usually a stone roll.
The object of the present invention is to further develop the Valmet Oy "Sym-Press II" press section.
The present invention is based on a great deal of experience that has been generated when using the "Sym-Press II" press section.
A feature of the basic version of the "Sym-Press II" is that only one side of the web is pressed against a smooth stone surface or a similar synthetic surface. This results in the web becomes asymmetric to a certain extent, and is undesirable regarding certain paper qualities.
A separate additional nip which is formed between the top stone roll and the bottom press roll has been used in association with "Sym-Press II". This additional nip is aimed at improving water removal and increasing the symmetry of the web by pressing both sides of the web against a smooth stone roll. The distance between the stone rolls will be quite long.
A problem in such a press section is the manner of directing the web from the first stone roll to the last additional nip, because the web tends to come loose from the lower felt carrying the same. The press section will also be quite bulky, which adds to the paper machine room expenses and makes the use of the press difficult. Another drawback is the very high frame section and its tendency to vibrate, partially due to the fact that it is necessary to support the bearings of the massive stone roll above the last nip and on the frame beams above the stone roll.
The above-described "Sym-Press II" that is equipped with an additional nip, is not often suitable for renovations of paper machines, due to bulkiness.